Method and apparatus for high-speed data transfer employing self-synchronizing quadrature amplitude modulation

ABSTRACT

A Quadrature Amplitude Modulation (QAM) method and apparatus including a QAM transmit modulator with at least one unbalanced mixer, which creates an asymmetric two-dimensional (2-D) QAM symbol constellation. The asymmetrical symbol constellation provides baseband symbol clock signal leakage sufficient to facilitate quick and simple baseband symbol clock recovery and signal channel compensation at the QAM receiver without significantly degrading the system bit-error rate (BER). While slightly degrading static BER, overall system performance is improved when considering baseband symbol clock recovery and received signal compensation for an imperfect signal channel. This allows QAM to be deployed in systems where QAM is otherwise prohibitively expensive and improves overall system performance for any existing QAM system application without additional bandwidth, cost or complexity.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/157,884 filed on Mar. 5, 2009, which isentirely incorporated herein by reference.

This application is also related to U.S. patent application Ser. No.12/399,859 filed Mar. 6, 2009, by Mark S. Olsson et al., entitled “PipeInspection System with Selective Image Capture,” the entire disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electronic information transfersystems and more particularly to a communication system for transferringdata in a pipe inspection system.

2. Description of the Related Art

Analog and digital Quadrature Amplitude Modulation (QAM) methods foramplitude modulating two symbol clocks phase-locked in quadrature havebeen known and used since the early days of signal processing and arewidely used today. For example, analog QAM is used to transfer thechroma component information in the 1953 National Television SystemCommittee (NTSC) and the 1963 Phase Alternating Line (PAL) standardtelevision signals and a 1977 Compatible QAM variation (C-QUAM) is stillused to transfer the stereo difference information in some AM stereoradio signals. More recently, a variety of digital QAM schemes(quantized QAM) were adapted for widespread use in cellular systems andfor other wireless applications, including the WiMAX and Wi-Fi 802.11standards.

Advantageously, digital QAM may be configured with Amplitude-ShiftKeying (ASK) to provide many data bits per symbol and thereby increasedata transfer rates in a channel without increasing Inter-SymbolInterference (ISI). Amplitude modulating two symbol clocks in quadrature(QAM) can be equivalently viewed as both amplitude modulating and phasemodulating a single symbol clock and each such modulation value(amplitude and phase) can be represented as a single point (symbol) onthe phase plane diagram, as is well-known in the art. For example, byusing two distinct amplitudes and four phase shift states for each ofthese amplitudes, a single symbol clock cycle can serve to carry onesymbol having eight states; equivalent to three bits of information. Inthis example, a 5 MHz channel baseband can transfer data at 15 Mb/s atthe expense of requiring a more robust method for reducing the impact ofnoise and increasing the Signal-to-Noise Ratio (SNR) to permit recoveryof the significantly higher number of discrete signal amplitudesinvolved in each symbol clock cycle.

Proper separation of the I(t) and Q(t) quadrature components of adigital or analog QAM signal requires the coherent demodulator signalphase at the receiver to be exactly in phase with the received QAMsignal carrier. Even a small demodulating phase error introducescrosstalk between the I(t) and Q(t) quadrature components recovered froma digital or analog QAM signal. Both symbol clock and carrier recoverysystems in a receiver attempt to derive information about timing fromthe received signal, often in a similar manner. While carrier recoveryis only necessary in a coherent demodulation system, symbol clockrecovery is required in all schemes, and accurate clock recovery isessential for reliable data transmission. Confusion often exists betweenclock and carrier recovery. Clock recovery attempts to synchronize thereceiver clock with the baseband symbol rate transmitter clock, whereascarrier recovery attempts to align the receiver local oscillator withthe transmitted carrier frequency.

Thus, symbol clock synchronization at the receiver must be handledsomehow in any QAM system. Any phase and frequency variations introducedby the channel must be removed at the receiver by properly tuning thesine and cosine components of the local QAM demodulator, which requiresa local symbol clock phase reference that is typically provided by someuseful version of a local Phase-Locked Loop (PLL). But this local phasereference must somehow be synchronized with the received QAM signalsymbol clock. For example, early analog QAM television systems transmita burst of the color subcarrier after each horizontal synchronizationpulse for local clock phase reference synchronization.

The QAM art has evolved in various ways to increase throughput andreliability. A typical QAM data communication system includes atransmitter, a receiver, and an unknown time-invariant channel in whicha complex-valued sequence of input data representing a series of symbolsselected from a complex symbol alphabet (also denominated a“constellation” on the complex I-Q plane or “phase plane”) are sentthrough the channel to be interpreted by the receiver. Conventional QAMsystems assume that channel noise is independent of input data andrelatively stationary. Some distortion of the transmitted signal istypical of non-ideal channel media including wired and wirelessconnections.

The QAM demodulator is by far the most complex element of the QAMsystem. The demodulator must detect the phase and amplitude of thereceived signal, decode each symbol based on the phase and amplitude ofthe baseband symbol clock and then finally convert the symbol data backto a serial stream. The baseband symbol clock must be recovered tocomplete the symbol demodulation. Clock recovery is a recurring problemwith any digital signal processing system.

The QAM art is replete with improvements intended to increase channeldata transfer capacity while reducing receiver cost and complexity.There is an undesirable level of complexity and overhead in conventionalQAM receivers for filtering signals and recovering baseband symbol clocksynchronization. In applications where channel bandwidth is limited,such as pipe inspection system channels with a handful of hard-wiredconductors, additional problems include correcting for a variable-lengthcopper channel and limiting camera-end hardware complexity to facilitatethe small package size necessary for movement inside pipes.

Practitioners in the art have proposed a wide variety of methodssimplifying the QAM carrier and clock recovery problem. For example, inU.S. Publ. Appl. No. 2009/0,147,839 A1, Grenabo discloses an improvedphase error detector for a QAM receiver but neither considers norsuggests any symbol constellation adjustments. Similarly, in U.S. Pat.No. 7,283,599 B1, Herbig discloses an improved phase error detector fora QAM receiver suitable for improving phase locking characteristics butneither considers nor suggests using an asymmetric symbol constellation.And, in U.S. Pat. No. 4,987,375, Wu et al. disclose a carrier lockdetector for a QAM system employing symbol detection ratios and usefulfor improved reliability at low SNR but neither consider nor suggest anysymbol constellation adjustments.

Practitioners in the art have also proposed a wide variety of methodsfor improving QAM system performance through manipulation of the symbolconstellations. For example, in U.S. Publ. Appl. No. 2008/0,317,168 A1,Yang et al. disclose an integer spreading rotation technique for shapingsymmetric QAM symbol constellations to enhance signal space diversitybut neither consider nor suggest techniques for improving basebandsymbol clock recovery at the receiver. These practitioners appear tofirmly believe that the QAM symbol constellation must be as symmetric aspossible about the phase plane origin to minimize the system Bit-ErrorRate (BER).

Some practitioners have found certain slight asymmetries in the QAMsymbol constellation to have some utility but have neither taught norsuggested using changes to the symbol constellation to improve basebandsymbol clock recovery in QAM system receivers. For example, O'Hara etal. (“Orthogonal-Coded Selective Mapping (OCSM) For OFDM Peak-To-AveragePower Reduction Without Side Information,” Proceeding of the SDR 04Technical Conference and Product Exposition. 2004) propose a selectivemapping (SM) method for reducing peak-to-average power (PAP) inOrthogonal Frequency Division Multiplexing (OFDM) systems that isachieved by introducing a very small asymmetry to the QAM subcarrierconstellations before scrambling. But O'Hara et al. take pains to pointout that this does not mean that the QAM subcarrier constellations areno longer zero-mean over time because the subsequent antipodalscrambling process returns the subcarrier symbol constellations tozero-mean symmetry again before transmission.

Other practitioners have suggested using a pilot tone in a QAM channelto improve channel estimation. For example, Tariq et al. (“EfficientImplementation Of Pilot-Aided 32 QAM For Fixed Wireless And Mobile ISDNApplications,” Vehicle Tech. Conf. Proc., 2000, VTC 2000-Spring Tokyo.2000 IEEE 51^(st), Vol. 1, pp. 680-684) discloses an improved QAM systemwhere a gap is created in the center of the information bearing signalspectrum and a pilot tone inserted therein before transmission. Tariq etal. neither teach nor suggest that their pilot tone has any relationshipto the QAM baseband symbol clock; in fact, they teach using the pilottone at the receiver only for the purpose of channel estimation andcompensation. In U.S. Pat. No. 3,813,598, Stuart discloses a pilot-toneaided QAM carrier recovery system that adds a pilot tone to the QAMtransmission either above or below the QAM modulator output spectrum,which may be recovered and used to deduce channel distortion effects atthe receiver, but Stuart neither considers nor suggests any manipulationof the symmetric QAM symbol constellation for baseband symbol clockrecovery. In U.S. Pat. No. 6,493,490 B1, Lin et al. disclose an improvedphase detector for carrier recovery in a dual-mode QAM/VSB (VestigialSideband) receiver system. Lin et al. discuss creating a pilot-toneaided Offset-QAM signal by first delaying the Q component by one half ofa symbol, thereby offsetting the Q rail, in time, from information onthe I rail, but neither consider nor suggest using an asymmetric QAMsymbol constellation. Hyun et al., (“Interleaved 5820 Code For InsertionOf Carrier And Clock Pilots In 64-QAM Systems,” IEEE ElectronicsLetters, Vol. 27, No. 18, pp. 1635-6, 29 Aug. 1991) disclose a methodfor selecting symbols from a symmetric diamond-shaped symbolconstellation to introduce a spectral null at the Nyquist frequency,thereby permitting the detection of a low-power clock pilot signalinserted at the null frequency, but neither consider nor suggest usingan asymmetric QAM symbol constellation.

SUMMARY OF THE INVENTION

This invention arises from the unexpectedly advantageous observationthat operating a Quadrature Amplitude Modulation (QAM) transmittermodulator with at least one unbalanced mixer, which creates anasymmetric two-dimensional (2-D) QAM symbol constellation, providesbaseband symbol clock signal leakage sufficient to facilitate quick andsimple baseband symbol clock recovery at the QAM receiver withoutsignificantly degrading the system Bit-Error Rate (BER). In fact, theQAM method of this invention flattens the system BER curve to reduce theSignal-to-Noise Ratio (SNR) required to provide lower BERs by as much asseveral decibels (dB). This is a profound and completely unexpectedobservation that has advantageous applications in many QAM systems,including (without limitation) pipe inspection systems, cell phonesystems, commercial broadcast systems, Wi-Fi systems and many others.

It is a purpose of this invention to provide QAM channel baseband symbolclock recovery that reduces the system BER, complexity and computationalload in certain SNR regions.

It is an advantage of this invention that it may be extended to anysystem generally relying on QAM methods to encode a transmitted signal.More specifically, the QAM method of this invention may be adapted toimprove the lower functional layers (the physical transmission,reception, media correction and timing recovery elements) in certain SNRregions of any data transmission and reception system using a variant ofQAM or any of its derivatives that employ two-dimensional (2-D) symbolconstellations, such as Orthogonal Frequency-Division Multiplexing(OFDM), Quotient Quadrature Amplitude Modulation (QQAM), etc. Except forthe improved BER in certain SNR regions, the QAM method of thisinvention does not affect the higher QAM system functional layers knownin the art, such as forward error correction coding, symbol scrambling,symbol mapping, etc.

It is an advantage of this invention that the effects of the QAM channelcharacteristics can be automatically corrected at the receiver withoutadditional receiver complexity or cost.

It is an advantage of this invention that, in a pipe inspection systemwith limited camera-transmitter space, the processing complexity isconstrained to the QAM receiver, reducing space and complexityrequirements for the camera-transmitter.

In one aspect, the invention is a method for transferring data throughthe signal channel including the steps of encoding the data to produce afirst baseband modulating signal I(t) and a second baseband modulatingsignal Q(t) whose amplitudes together represent a time series of complexsymbols (I, Q) each selected from a two-dimensional (2-D) constellationof symbols distributed on the phase plane about the origin such that atleast one of the baseband modulating signals has a substantiallynon-zero mean amplitude; multiplying the first baseband modulatingsignal I(t) by a first baseband symbol clock signal to produce a firstmodulation product signal and multiplying the second baseband modulatingsignal Q(t) by a second baseband symbol clock signal to produce a secondmodulation product signal, where the phases of the first and secondbaseband symbol clock signals are generally fixed in quadrature; summingthe first and second modulation product signals to produce a transmitteroutput signal; coupling the transmitter output signal through the signalchannel to the data receiver; and demodulating the first and secondmodulation product signals at the data receiver to recover the series ofcomplex symbols (I, Q).

In another aspect, the invention is a communication system including adata transmitter having an input for accepting data, a QuadratureAmplitude Modulation (QAM) encoder coupled to the data input forproducing, responsive to the data, a first baseband modulating signalI(t) and a second baseband modulating signal Q(t) whose amplitudestogether represent a time series of complex symbols (I, Q) each selectedfrom a two-dimensional (2-D) constellation of symbols distributed on thephase plane about the origin such that at least one of the basebandmodulating signals has a substantially non-zero mean amplitude, a QAMmodulator coupled to the QAM encoder for multiplying the first basebandmodulating signal I(t) by a first baseband symbol clock signal toproduce a first modulation product signal and multiplying the secondbaseband modulating signal Q(t) by a second baseband symbol clock signalto produce a second modulation product signal, where the phases of thefirst and second baseband symbol clock signals are generally fixed inquadrature, and for summing the first and second modulation productsignals to produce a transmitter output signal, and an output forcoupling the transmitter output signal to a signal channel; and a datareceiver having a signal input coupled to the signal channel foraccepting the transmitter output signal, and a QAM demodulator coupledto the signal input for recovering the series of complex symbols (I, Q)from the first and second modulation product signals.

In yet another aspect, the invention is a data modulator for a videotransmitter including an input for accepting data; a QuadratureAmplitude Modulation (QAM) encoder coupled to the data input forproducing, responsive to the data, a first baseband modulating signalI(t) and a second baseband modulating signal Q(t) whose amplitudestogether represent a time series of complex symbols (I, Q) each selectedfrom a two-dimensional (2-D) constellation of symbols distributed on thephase plane about the origin such that at least one of the basebandmodulating signals has a substantially non-zero mean amplitude; and aQAM modulator coupled to the QAM encoder for multiplying the firstbaseband modulating signal I(t) by a first baseband symbol clock signalto produce a first modulation product signal and multiplying the secondbaseband modulating signal Q(t) by a second baseband symbol clock signalto produce a second modulation product signal, where the phases of thefirst and second baseband symbol clock signals are generally fixed inquadrature, and for summing the first and second modulation productsignals to produce a transmitter output signal.

In one embodiment, the invention is a pipe inspection system including avideo transmitter having a video camera adapted to produce video data,and a QAM modulator coupled to the video camera, including a symbolencoder for producing, responsive to the video data, a first basebandmodulating signal I_(T)(t) and a second baseband modulating signalQ_(T)(t) whose amplitudes together represent a time series of complextransmitter symbols (I_(T), Q_(T)) each selected from a two-dimensional(2-D) constellation of symbols distributed on the phase plane about theorigin such that at least one of the baseband modulating signals has asubstantially non-zero mean amplitude, a baseband symbol clockoscillator for producing first and second baseband symbol clock signalsgenerally fixed in quadrature, a dual multiplier coupled to the symbolencoder and baseband symbol clock oscillator for multiplying the firstbaseband modulating signal I_(T)(t) by the first baseband symbol clocksignal to produce a first modulation product signal and for multiplyingthe second baseband modulating signal Q_(T)(t) by the second basebandsymbol clock signal to produce a second modulation product signal, asummer coupled to the dual multiplier for summing the first and secondmodulation product signals to produce a transmitter output signal, and afilter coupled to the summer for producing a filtered transmitter outputsignal; a mechanical cable assembly coupled to the video transmitter forurging the video transmitter through a pipe under inspection andincluding an electrical conductor coupled to the QAM modulator foraccepting the filtered transmitter output signal; and a video receiverhaving a signal conditioner coupled to the electrical conductor forproducing a baseband receiver input signal representing the filteredtransmitter output signal, a QAM demodulator coupled to the signalconditioner, including a baseband symbol clock detector for detectingthe first baseband symbol clock signal from the receiver input signal, abaseband symbol clock recovery oscillator coupled to the baseband symbolclock detector for producing a first recovered baseband symbol clocksignal generally synchronized with the first baseband symbol clocksignal and for producing a second recovered baseband symbol clock signalgenerally fixed in quadrature with the first recovered baseband symbolclock signal, a dual multiplier coupled to the baseband symbol clockrecovery oscillator for multiplying the baseband receiver input signalby the first and second recovered baseband symbol clock signals toproduce first and second demodulation product signals, respectively, adual filter coupled to the dual multiplier for producing, responsive tothe first and second demodulation product signals respectively, firstand second baseband demodulated signals, I_(R)(t) and Q_(R)(t), whoseamplitudes together represent a time series of complex receiver symbols(I_(R), Q_(R)), and a decoder coupled to the QAM demodulator forrecovering the video data from the first and second demodulated signals,I_(R)(t) and Q_(R)(t), and a video display coupled to the QAMdemodulator for producing images responsive to the video data.

The foregoing, together with other objects, features and advantages ofthis invention, can be better appreciated with reference to thefollowing specification, claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, in which like referencedesignations represent like features throughout the several views andwherein:

FIG. 1 is a schematic diagram illustrating a typical 256-QAM Type IIIsymmetrical symbol constellation from the prior art;

FIG. 2 is a graph of a typical signal spectrum from a typical 256-QAMdata communication system from the prior art using the symbolconstellation of FIG. 1 while transmitting pseudorandom data at twomillion symbols per second.

FIG. 3 is a schematic diagram illustrating a typical QAM datacommunication system from the prior art;

FIG. 4 is a schematic diagram illustrating an exemplary embodiment of aQAM data communication system of this invention;

FIG. 5A is a schematic diagram illustrating an exemplary 256-QAMasymmetrical symbol constellation suitable for use in the system of thisinvention;

FIG. 5B is a schematic diagram illustrating an alternative 256-QAMasymmetrical symbol constellation suitable for use in the system of thisinvention;

FIG. 6 is a flowchart illustrating an exemplary method of this inventionfor transferring data through a signal channel;

FIG. 7 is a graph illustrating the theoretical Bit Error Rate (BER)characteristics for several 256-QAM system embodiments from the priorart;

FIG. 8A is a block diagram illustrating a first embodiment of a QAMmodulator and demodulator assuming ideal demodulation;

FIG. 8B is a block diagram illustrating a second embodiment of a QAMmodulator and demodulator using the undistorted modulator basebandsymbol clock signal at the demodulator;

FIG. 8C is a block diagram illustrating a third embodiment of a QAMmodulator and demodulator from the prior art using a cable andpreamplifier channel and a Phase-Locked Loop (PLL) for demodulatorbaseband symbol clock recovery;

FIG. 8D is a block diagram illustrating a fourth embodiment of a QAMmodulator and demodulator from the prior art using a cable andpreamplifier channel and an exotic means for demodulator baseband symbolclock recovery;

FIG. 9 is a graph illustrating the theoretical BER characteristics forseveral 256-QAM system embodiments of this invention;

FIG. 10A is a block diagram illustrating a first embodiment of a QAMmodulator and demodulator of this invention using the undistortedmodulator baseband symbol clock signal at the demodulator;

FIG. 10B is a block diagram illustrating a second embodiment of a QAMmodulator and demodulator of this invention a cable and preamplifierchannel and a delayed modulator baseband symbol clock signal at thedemodulator;

FIG. 10C is a block diagram illustrating a third embodiment of a QAMmodulator and demodulator of this invention using a cable andpreamplifier channel and a PLL for demodulator baseband symbol clockrecovery;

FIG. 11 is a graph illustrating the baseband transmitter output signalin the time domain from a 256-QAM system embodiment using thesymmetrical Type III symbol constellation from FIG. 1;

FIG. 12 is a graph illustrating the baseband transmitter output signalin the time domain from a 256-QAM system embodiment using the exemplaryasymmetrical Type III symbol constellation of this invention from FIG.5A;

FIG. 13 is a graph illustrating the signal of FIG. 11 in the spectraldomain;

FIG. 14 is a graph illustrating the signal of FIG. 12 in the spectraldomain;

FIG. 15 is a graph illustrating the theoretical BER characteristics forseveral 256-QAM system embodiments of this invention;

FIG. 16 is a graph comparing the theoretical BER characteristics fromFIG. 15 to several BER characteristics from FIG. 7;

FIG. 17 is a perspective diagram illustrating an exemplary embodiment ofa pipe mapping system of this invention incorporating the data transfersystem of this invention;

FIG. 18 is a block diagram illustrating the electronic portion of thesystem of FIG. 17 a method for transferring a video signal through thesignal channel that processes the flow of image data, camera controldata, distance counter data, user interface data, and displayinformation; and

FIG. 19 is a flowchart illustrating an exemplary method of thisinvention for transferring a video signal through signal channel in apipe inspection system of FIG. 17.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Quadrature Amplitude Modulators:

Digital Quadrature Amplitude Modulation (QAM) schemes may be betterunderstood with reference to the well-known two-dimensional (2-D) QAMsymbol constellation diagram showing the QAM symbol states eachrepresented as two (I and Q) amplitudes mapped as points on a complexI-Q plane (herein also denominated “the phase plane”). These 2-D symbolconstellation mappings may also be represented as a radius amplitude anda phase angle measured from the phase plane origin, for example, but aregenerally understood to represent two amplitudes measured from the phaseplane origin along the respective orthogonal I and Q axes. In QAMsystems, the 2-D symbol constellation states are often arranged in asymmetrical square grid with equal vertical and horizontal spacing,although many other symmetrical configurations are known to be useful(e.g., Cross-QAM). As digital data are usually binary, the number ofstates (points or symbols) in the constellation is usually a power oftwo. Because the digital QAM symbol constellation is usually square, thecommon grids are numbered in powers of four; providing us with 16-QAM,64-QAM, and 256-QAM systems, etc. These well-known square QAM symbolconstellations go as high as 4096-QAM, which provides 4 kb/symbol with64 different amplitude levels in both I and Q. With a higher-orderconstellation, the QAM system can transmit more bits per symbol but thepoints are more closely spaced for the same mean constellation energyand are thus more susceptible to noise and other corruption, producinghigher bit error rates. Thus, higher-order QAM delivers more data lessreliably than lower-order QAM for a given mean constellation energy.

These square symbol constellations are also denominated Type III QAMconstellations. A Type I QAM symbol constellation has states arrangedsymmetrically about the phase plane origin along equally-spaced radiallines extending out from the phase plane origin with the same number ofstates in each of several concentric circles. A Type II QAM symbolconstellation is similar to the Type I but reduces the number of stateson the inner concentric circles (because phase angles detection is lessaccurate at lower amplitudes) while retaining symmetry about the phaseplane origin. Type III QAM symbol constellations are square and centeredon the phase plane origin. Each state is a 2-D value (I, Q) representingone of “n” amplitudes in I-space and one of “n” amplitudes in Q-space.It graphically represents each QAM symbol with amplitudes alone and theimplicit phase angle defined on the phase plane by arctan(I/Q) arisesonly because of the 2-D representation of the amplitude pair (I, Q).

FIG. 1 is a schematic diagram illustrating a typical 256-QAM Type IIIconstellation 100 from the prior art. In this “square” constellation,the I-space values are represented as sixteen amplitudes ranging from−7.5 units to +7.5 units spaced along the I-axis 102 and the Q-spacevalues are represented as the same sixteen amplitude values spaced alongthe Q-axis 104. The I-space values are equally spaced by 1.0 unit asexemplified by the spacing 106 and the Q-space values are equally spacedby the same amount as exemplified by the spacing 108. I-axis 102 andQ-axis 104 cross orthogonally at the phase plane origin 110. Each symbolstate is represented as a finite region about a point (I, Q), asexemplified by the symbol state 112, which represents eight bits ofdata; four bits encoded in each of the sixteen amplitude values reservedfor I and four bits encoded in each of the sixteen amplitude valuesreserved for Q. Constellation 100 is disposed with the I-space andQ-space ranges symmetrically centered about phase plane origin 110 suchthat any I-space baseband modulating signal I(t) and Q-space basebandmodulating signal Q(t) together representing a random time-series of (I,Q) symbols will both have zero-mean amplitudes (no DC components) toeliminate clock leakage in the manner well-known in the art.

FIG. 2 is a graph of a typical 256-QAM signal spectrum 114 from atypical 256-QAM data transmission system from the prior art (e.g., FIG.3) using symbol constellation 100 (FIG. 1) while transmittingpseudorandom data at two million symbols per second (using a basebandsymbol clock frequency of 2 MHz). As seen in FIG. 2, there is noevidence of any additional signal at the baseband symbol clock frequency116 or anything else sufficiently “obvious” to allow reconstruction ofthe baseband symbol clock timing at the receiver. Thus, the QAMreception problem remains complex and unreliable, as is well-known.Various timing recovery tricks are known in the art, ranging from“early-late” guesses (using a predictor-corrector method) to a combinedphase-frequency detector, for example. Note that channel estimation andcompensation also must be accomplished using the only availableinformation, which is limited to wide-band or narrowband powerestimation. Various channel estimation solutions known in the artinclude injecting separate “pilot” signals and other similarly complextechniques, for example. These constraints add unwanted complexity toany QAM receiver.

FIG. 3 is a schematic diagram illustrating a typical QAM communicationsystem 200 from the prior art, including a QAM data transmitter 202 anda QAM data receiver 204 coupled by a signal channel 206. In FIG. 3, adata input 208 accepts a stream of incoming data 210 for processing andtransmission. Incoming data 210 is routed to the encoder 212 forrandomizing, interleaving, error-correction and other high-levelencoding, for example. The randomized data 214 is then routed to the QAMencoder 216, which separates the data into the two baseband modulatingsignals, I_(T)(t) 218 and Q_(T)(t) 220, which together represent atime-series of complex transmitter symbols (I_(T), Q_(T)) (not shown)selected according to the mapping of each pair of four-bit sequences ofrandomized data 214 onto constellation 100 (FIG. 1). This mapping isimportant because each complex transmitter symbol (I_(T), Q_(T))represents eight bits in this example. A channel deficiency is mostlikely to cause a symbol error by incorrectly assessing thecorresponding received symbol as the one immediately adjacent thetransmitted symbol in constellation 100 (FIG. 1), so encoder 212 mustencode incoming data 210 to minimize the overall bit error rate arisingfrom simple symbol errors. Simple binary coding, for example, is notvery robust against bit errors (e.g., a single symbol step from 01111111to 10000000 collects eight bit errors) so the symbol mapping strategymust be chosen carefully, as is well known. A “Gray” code is useful andcommonly used.

The two baseband modulating signals, I_(T)(t) 218 and Q_(T)(t) 220, areaccepted by a QAM modulator 222 that includes an I-modulator 224 and aQ-modulator 226 embodied as a dual multiplier. I-modulator 224 modulatesa zero-degree-phase baseband symbol clock signal 228 from the basebandsymbol clock oscillator 229 by multiplying it with baseband modulatingsignal I_(T)(t) 218 to produce an I-modulation product signal 230 andQ-modulator 226 modulates a ninety-degree-phase baseband symbol clocksignal 232 by multiplying it with baseband modulating signal Q_(T)(t)220 to produce a Q-modulation product signal 234. A summer 236 then addsI-modulation product signal 230 and Q-modulation product signal 234 inthe usual manner to produce a transmitter output signal 238, which, inthis example, is filtered and conditioned at the filter and driverassembly 240 to produce a filtered transmitter output signal 242 that isconditioned for transfer through the physical transmission medium insignal channel 206 to QAM receiver 204. Zero-degree-phase andninety-degree-phase baseband symbol clock signals 228 and 232 are saidto be generally fixed in quadrature because they are phase-locked to oneanother with a 90-degree phase difference in the usual manner. Signalchannel 206 may include conductive wiring, optical fiber, modulatedradio frequency or optical signals in free space, or any other usefulchannel means known in the art, for example. Filter and driver assembly240 may include an additional modulator(s) (not shown) forreconditioning transmitter output signal 238 as a modulation product ofanother carrier signal more suited to the signal channel medium, forexample.

Continuing with FIG. 3, a signal conditioner 244 in QAM data receiver204 accepts from signal channel 206 a channel signal 246 that representsfiltered transmitter output signal 242 in some manner, depending onparticular channel characteristics, added noise, and the like. Signalconditioner 244 may include an additional demodulator(s) (not shown) forrecovering the baseband component of channel signal 246 when usinganother carrier signal more suited to the signal channel medium, forexample. Signal conditioner 244 restores the signal level and providesany additional (usually analog) reconditioning necessary to produce abaseband receiver input signal 248. From here, receiver input signal 248takes two paths; the first taking it to a baseband symbol clock detector250 for baseband symbol clock timing recovery and the second taking itto an equalization and correction circuit 252 for any additionalprocessing necessary to correct for noise, intersymbol interference(ISI) and other unwanted effects of the trip through signal channel 206.Baseband symbol clock detector 250 includes a baseband symbol clockrecovery oscillator 254 that produces a zero-degree-phase recoveredbaseband symbol clock signal 256 and a ninety-degree-phase recoveredbaseband symbol clock signal 258, which are generally fixed inquadrature and respectively synchronized with baseband symbol clocksignals 228 and 232 above. Equalization and correction circuit 252produces a baseband receiver input signal 260 that (as much as possible)represents the recovery of transmitter output signal 238.

The baseband receiver input signal 260 from equalization and correctioncircuit 252 is routed to the QAM demodulator 262 for recovery of the twobaseband demodulated signals, I_(R)(t) 264 and Q_(R)(t) 266, togetherrepresenting a time series of complex receiver symbols (I_(R), Q_(R))that (as much as possible) represent the recovery of the initialtime-series of complex transmitter symbols (I_(T), Q_(T)) discussedabove. This is accomplished by an I-demodulator 268 and a Q-demodulator270 embodied as a dual multiplier. I-demodulator 268 demodulatesbaseband receiver input signal 260 by multiplying it withzero-degree-phase recovered baseband symbol clock signal 256 to producean I-demodulation product signal 272 and Q-demodulator 270 demodulatesbaseband receiver input signal 260 by multiplying it withninety-degree-phase recovered baseband symbol clock signal 258 toproduce a Q-demodulation product signal 274. I-demodulation productsignal 272 is passed through a first low-pass filter 276 to recoverbaseband demodulated signal I_(R)(t) 264 and Q-demodulation productsignal 274 is passed through a second low-pass filter 278 to recoverbaseband demodulated signal Q_(R)(t) 266 in the usual manner. From QAMdemodulator 262, both baseband demodulated signals, I_(R)(t) 264 andQ_(R)(t) 266 are presented to the QAM decoder 280 for reversal of the2-D constellation mapping process performed in QAM encoder 216 anddiscussed above to produce the recovered randomized data 282. Finally,in the decoder 284, the randomizing, interleaving, error-correction andother high-level encoding processing performed in encoder 212 anddiscussed above is reversed to produce a stream of output data 286corrected for errors where possible and timed according to a bit rateclock signal 288 from baseband symbol clock detector 250. A feedbackline 290 to equalization and correction circuit 252 permits recoveryoptimization by adjusting the conditioning of receiver input signal 248to minimize errors detected and corrected in recovered randomized data282 by decoder 284, for example.

To appreciate the detailed operation of QAM communication system 200(FIG. 3) consider a simple QAM encoding example based on 256-QAM TypeIII constellation 100 (FIG. 1). Referring to FIG. 3, consider thedetails of passing several complex transmitter symbols (I_(T), Q_(T))through QAM communication system 200 starting with QAM modulator 222 andassuming that transmitter output signal 238 passes through signalchannel 206 to QAM data receiver 204 with perfect fidelity.

As QAM operates with quantized amplitudes, assume that the I-axis 102and Q-axis 104 range from −7.5 units to 7.5 units, in 1.0 unit steps.For example, the units may represent volts or any other physicaldenomination suitable to the application. This arrangement therebyprovides sixteen amplitudes along each axis that may be convenientlymapped (in any sequence) to the sixteen available four-bit binarysequences ranging from 0000 to 1111, consistent with the abovediscussion. Assume for this illustration that the stream of incomingdata 210 is sixteen bits long and may be mapped by constellation 100 tothe following two exemplary complex transmitter symbols (I_(T), Q_(T))over two complete four-part baseband symbol clock cycles (using logicalamplitude units):

-   -   Complex transmitter symbols (I_(T), Q_(T)): (+1.5, −6.5) and        (−3.5, +5.5)

So, the two baseband modulating signals, I_(T)(t) 218 and Q_(T)(t) 220have the following amplitudes over the two four-part baseband symbolclock cycles:

-   -   First baseband modulating signal, I_(T)(t) 218:        +1.5,+1.5,+1.5,+1.5,−3.5,−3.5,−3.5,−3.5    -   Second baseband modulating signal, Q_(T)(t) 220:        −6.5,−6.5,−6.5,−6.5,+5.5,+5.5,+5.5,+5.5

Assuming that, in QAM modulator 222, baseband symbol clock signal 228 isa square wave with either a 0 or 1 logical amplitude, the followingsymbol clock signal values describe the two complete four-part symbolclock cycles mapping onto these two complex transmitter symbols (I_(T),Q_(T)):

-   -   Zero-degree-phase baseband symbol clock signal 228 (I-clock):        +1,+1,−1,−1,+1,+1,−1,−1    -   Ninety-degree-phase baseband symbol clock signal 232 (Q-clock):        −1,+1,+1,−1,−1,+1,+1,−1

After the multiplications in I-modulator 224 and Q-modulator 226, theresulting modulation product signal amplitudes over the two four-partbaseband symbol clock cycles are:

-   -   I-modulation product signal 230:        +1.5,+1.5,−1.5,−1.5,−3.5,−3.5,+3.5,+3.5    -   Q-modulation product signal 234:        +6.5,−6.5,−6.5,+6.5,−5.5,+5.5,+5.5,−5.5

When added together at summer 236, the amplitude of transmitter outputsignal 238 over the two four-part baseband symbol clock cycles is:

-   -   Transmitter output signal 238:        +8.0,−5.0,−8.0,+5.0,−9.0,+2.0,+9.0,−2.0

In this example, transmitter output signal 238 is also the receiverinput signal 248 arriving at QAM data receiver 204 from which twocomplex receiver symbols (I_(R), Q_(R)) must be recovered and decoded torecover the stream of incoming data 210 without error if possible.

-   -   Receiver input signal 248:        +8.0,−5.0,−8.0,+5.0,−9.0,+2.0,+9.0,−2.0

Assuming that zero-degree-phase recovered baseband symbol clock signal256 can be precisely synchronized with zero-degree-phase baseband symbolclock signal 228 in QAM data transmitter 202, then baseband symbol clockrecovery oscillator 254 provides the following logical amplitudes overtwo complete four-part recovered baseband symbol clock cycles:

-   -   Zero-degree-phase recovered baseband symbol clock signal 256:        +1,+1,−1,−1,+1,+1,−1,−1    -   Ninety-degree-phase recovered baseband symbol clock signal 258:        −1,+1,+1,−1,−1,+1,+1,−1

Thus, after the multiplications in I-demodulator 268 and Q-demodulator270, the following two demodulation product signals are producedcomplete four-part recovered baseband symbol clock cycles:

-   -   I-demodulation product signal 272:        +8.0,−5.0,+8.0,−5.0,−9.0,+2.0,−9.0,+2.0    -   Q-demodulation product signal 274:        −8.0,−5.0,−8.0,−5.0,+9.0,+2.0,+9.0,+2.0

Passing each of these two product signals through their respectivelow-pass filters 276 and 278 can be assumed to produce a average valueover each full baseband symbol clock cycle, thereby producing thefollowing logical amplitude averages for the two baseband demodulatedsignals, I_(R)(t) 264 and Q_(R)(t) 266 over two complete recoveredbaseband symbol clock cycles:

-   -   First baseband demodulated signal I_(R)(t) 264: 6.0/4=+1.5,        −14.0/4=−3.5    -   Second baseband demodulated signal Q_(R)(t) 266: −26.0/4=−6.5,        22.0/4=+5.5    -   Complex receiver symbols (I_(R), Q_(R)): (+1.5, −6.5) and (−3.5,        +5.5)

Finally, in QAM decoder 280 and decoder 284, the two complex receiversymbols (I_(R), Q_(R)) are decoded with reference to constellation 100(FIG. 1) to obtain the stream of output data 286 that represents(ideally without error) original data stream 210. In this example, twosymbols at the channel symbol rate serves to transmit and correctlyreceive sixteen bits of information. In a practical application,assuming adequate timing recovery means, the received signal may besampled four times during the baseband symbol clock cycle to retrievethe data correctly.

Improving QAM Clock Recovery:

Notice that some form of timing recovery must be performed in basebandsymbol clock detector 250 to recover baseband symbol clock signals 256and 258 as well as bit rate clock signal 288. The QAM receiver clockrecovery function is expensive in terms of computing (and electrical)power and parts cost. The reason for this may be appreciated withreference to FIG. 2. Note that 256-QAM signal spectrum 114 in FIG. 2 isnulled at 0 Hz (DC) and 4 MHz (twice the 2 MHz symbol clock rate), buthas no prominent component at the symbol clock rate 116 so that basebandsymbol clock recovery is feasible only by applying exotic statisticalmethods to receiver input signal 248. But these exotic computationalcomponents are expensive. So, although QAM data communication system 200provides some utility and QAM data transmitter 202 alone is relativelyinexpensive, QAM data receiver 204 can be too complex and expensive forsimple applications, such as pipe inspection systems, for example.

FIG. 4 is a schematic diagram illustrating an exemplary QAM datacommunication system embodiment 300 of this invention, including a QAMdata transmitter 302 and a QAM data receiver 304 coupled through asignal channel 306. In this embodiment, many of the transmitterfunctions in QAM data transmitter 302 are embodied as software (orfirmware) programs in a Digital Signal Processor (DSP) 307 withprogramming adapted to accept a stream of incoming data 310 forprocessing and transmission. Incoming data 310 is routed to the encoder312 for randomizing, interleaving, error-correction and other high-levelencoding, for example. The randomized data 314 is then routed to the QAMencoder 316, which separates the data into the two baseband modulatingsignals, I_(T)(t) 318 and Q_(T)(t) 320, which together represent atime-series of complex transmitter symbols (I_(T), Q_(T)) (not shown)selected according to the mapping of each pair of four-bit sequences ofrandomized data 314 onto an asymmetric symbol constellation of thisinvention exemplified by the 256-QAM asymmetrical symbol constellation400 shown in FIG. 5A and by the 256-QAM asymmetrical symbolconstellation 500 shown in FIG. 5B.

FIG. 5A is a schematic diagram illustrating an exemplary 256-QAMasymmetrical symbol constellation 400 of this invention suitable for usein QAM communication system 300 (FIG. 4). In this “asymmetric”constellation, the I-space values are represented as sixteen amplitudesranging from −2.5 units to +12.5 units spaced along the I-axis 402 andthe Q-space values are represented as the same sixteen amplitudesranging from −7.5 units to +7.5 units spaced along the Q-axis 404. TheI-space values are equally spaced by 1.0 units as exemplified by thespacing 406 and the Q-space values are equally spaced by the same amountas exemplified by the spacing 408. I-axis 402 and Q-axis 404 crossorthogonally at the phase plane origin 410. Each symbol state isrepresented as a finite region about a point (I, Q), as exemplified bythe symbol state 412, which represents eight bits of data; four bitsencoded in each of the sixteen amplitude values reserved for I and fourbits encoded in each of the sixteen amplitude values reserved for Q.Constellation 400 is disposed with the I-space and Q-space rangesasymmetrically about phase plane origin 410 such that any I-spacebaseband modulating signal I(t) and Q-space baseband modulating signalQ(t) together representing a random time-series of (I, Q) symbols willhave mean amplitudes (DC components) that are substantially non-zero(+5.0 units in I-space for constellation 400) for I-space basebandmodulating signal I(t) and substantially zero for Q-space basebandmodulating signal Q(t). FIG. 5B is a schematic diagram illustrating analternative 256-QAM asymmetrical symbol constellation 500 suitable foruse in QAM communication system 300 (FIG. 4). In this “asymmetric”constellation, the I-space values are represented as sixteen amplitudesranging from +0.5 units to +15.5 units spaced along the I-axis 502 andthe Q-space values are represented as the same sixteen amplitudesranging from −7.5 units to +7.5 units spaced along the Q-axis 504. TheI-space values are equally spaced by 1.0 units as exemplified by thespacing 506 and the Q-space values are equally spaced by the same amountas exemplified by the spacing 508. I-axis 502 and Q-axis 504 crossorthogonally at the phase plane origin 510. Each symbol state isrepresented as a finite region about a point (I, Q), as exemplified bythe symbol state 512, which represents eight bits of data; four bitsencoded in each of the sixteen amplitude values reserved for I and fourbits encoded in each of the sixteen amplitude values reserved for Q.Constellation 500 is disposed with the I-space and Q-space rangesasymmetrically about phase plane origin 510 such that any I-spacebaseband modulating signal I(t) and Q-space baseband modulating signalQ(t) together representing a random time-series of (I, Q) symbols willhave mean amplitudes (DC components) that are substantially non-zero(+8.0 units in I-space for constellation 500) for I-space basebandmodulating signal I(t) and substantially zero for Q-space basebandmodulating signal Q(t). The non-zero DC bias of at least one of the twobaseband modulating signals is an important element of the system ofthis invention and either or both of the two baseband modulating signalsmay be biased to create a suitable asymmetric constellation inaccordance with these teachings.

Returning to FIG. 4, a constellation bias signal 321 is shown as aninput to QAM encoder 316 to illustrate the method for shifting thesymbol constellation exemplified by constellation 400, which may bethought of as adding a DC bias to either or both baseband modulatingsignals, I_(T)(t) 318 and Q_(T)(t) 320 during the encoding process inQAM encoder 316, for example. This facilitates using constellation biassignal 321 to adjust the asymmetric constellation exemplified byconstellation 400 in response to channel type and conditions or forother purposes, for example.

The two baseband modulating signals, I_(T)(t) 318 and Q_(T)(t) 320, areaccepted by a QAM modulator 322 that includes an I-modulator 324 and aQ-modulator 326 embodied as a dual multiplier. I-modulator 324 modulatesa zero-degree-phase baseband symbol clock signal 328 from the basebandsymbol clock oscillator 329 by multiplying it with baseband modulatingsignal I_(T)(t) 318 to produce an I-modulation product signal 330 andQ-modulator 326 modulates a ninety-degree-phase baseband symbol clocksignal 332 from baseband symbol clock oscillator 329 by multiplying itwith baseband modulating signal Q_(T)(t) 320 to produce a Q-modulationproduct signal 334. A summer 336 then adds I-modulation product signal330 and Q-modulation product signal 334 in the usual manner to produce adigital transmitter output signal 337, which is then converted to ananalog transmitter output signal 338 by the digital-to-analog converter339. Transmitter output signal 338 is filtered and conditioned at thefilter and driver assembly 340 to produce a filtered transmitter outputsignal 342 that is conditioned for transfer through the physicaltransmission medium in signal channel 306 to QAM receiver 304.Zero-degree-phase and ninety-degree-phase baseband symbol clock signals328 and 332 are said to be generally fixed in quadrature because theyare phase-locked to one another with a 90-degree phase difference in theusual manner. Signal channel 306 may include conductive wiring, opticalfiber, modulated radio frequency or optical signals in free space, orany other useful channel means known in the art, for example. Filter anddriver assembly 340 may include an additional modulator(s) (not shown)for reconditioning transmitter output signal 338 as a modulation productof another carrier signal more suited to the signal channel medium, forexample.

Continuing with FIG. 4, a signal conditioner 344 in QAM data receiver304 accepts from signal channel 306 a channel signal 346 that representsfiltered transmitter output signal 342 in some manner, depending onparticular channel characteristics, added noise, and the like. Signalconditioner 344 may include an additional demodulator(s) (not shown) forrecovering the baseband component of channel signal 346 when usinganother carrier signal more suited to the signal channel medium, forexample. Signal conditioner 344 restores the signal level and providesany additional (usually analog) reconditioning necessary to produce abaseband receiver input signal 348, which may now be converted back to adigital receiver input signal 349 by means of an analog-to-digitalconverter 351, which may be synchronized with baseband symbol clockdetector 350 substantially as shown. In this embodiment, many of thereceiver functions in QAM data receiver 304 are embodied as software (orfirmware) programs in a Digital Signal Processor (DSP) 359 withprogramming adapted to accept the digital receiver input signal 349 fordecoding and processing. In addition to the functional elements shownFIG. 4, DSP 359 may also embrace portions of baseband symbol clockdetector 350. Most remaining complexity in QAM data receiver 304 isfound in signal conditioner 344 and the remainder of baseband symbolclock detector 350. But baseband symbol clock detector 350 may now beimplemented as a “simple” Phase Locked Loop (PLL) circuit, for example,because of the asymmetrical symbol constellation 400 used in QAMtransmitter 302, for the reasons discussed herein below in connectionwith FIGS. 15-16. Notice that using DSP 359 in QAM data receiver 304 andthe simple PLL implementation of provides a simple and cost effectiveembodiment of the receiving element of this invention, thereby meetingthe primary purpose of the system of this invention. This allows QAMtechniques to be applied in a much more cost effective manner thanpreviously known, making QAM feasible for applications for which it waspreviously cost prohibitive.

Continuing with the remainder of FIG. 4, from signal conditioner 344,receiver input signal 348 takes two paths; the first taking it to abaseband symbol clock detector 350 for baseband symbol clock timingrecovery and the second taking it to analog-to-digital converter 351 fordigitization to produce digital receiver input signal 349, which ispresented to an equalization and correction circuit 352 for anyadditional processing necessary to correct for noise, intersymbolinterference (ISI) and other unwanted effects of the trip through signalchannel 306. Baseband symbol clock detector 350 includes a basebandsymbol clock recovery oscillator 354 that produces a zero-degree-phaserecovered baseband symbol clock signal 356 and a ninety-degree-phaserecovered baseband symbol clock signal 358, which are generally fixed inquadrature and respectively synchronized with baseband symbol clocksignals 328 and 332 above.

Equalization and correction circuit 352 produces a baseband receiverinput signal 360 that (as much as possible) represents the recovery oftransmitter output signal 338. The baseband receiver input signal 360from equalization and correction circuit 352 is routed to the QAMdemodulator 362 for recovery of the two baseband demodulated signals,I_(R)(t) 364 and Q_(R)(t) 366, together representing a time series ofcomplex receiver symbols (I_(R), Q_(R)) that (as much as possible)represent the recovery of the initial time-series of complex transmittersymbols (I_(T), Q_(T)) discussed above. This is accomplished by anI-demodulator 368 and a Q-demodulator 370 embodied as a dual multiplier.I-demodulator 368 demodulates baseband receiver input signal 360 bymultiplying it with zero-degree-phase recovered baseband symbol clocksignal 356 to produce an I-demodulation product signal 372 andQ-demodulator 370 demodulates baseband receiver input signal 360 bymultiplying it with ninety-degree-phase recovered baseband symbol clocksignal 358 to produce a Q-demodulation product signal 374.I-demodulation product signal 372 is passed through a first low-passfilter 376 to recover baseband demodulated signal I_(R)(t) 364 andQ-demodulation product signal 374 is passed through a second low-passfilter 378 to recover baseband demodulated signal Q_(R)(t) 366 in theusual manner. From QAM demodulator 362, both baseband demodulatedsignals, I_(R)(t) 364 and Q_(R)(t) 366 are presented to the QAM decoder380 for reversal of the 2-D constellation mapping process performed inQAM encoder 316 and discussed above to produce the recovered randomizeddata 382. Finally, in the decoder 384, the randomizing, interleaving,error-correction and other high-level encoding processing performed inencoder 312 and discussed above is reversed to produce a stream ofoutput data 386 corrected for errors where possible and timed accordingto a bit rate clock signal 388 from baseband symbol clock detector 350.A feedback line 390 to equalization and correction circuit 352 permitsrecovery optimization by adjusting the conditioning of receiver inputsignal 348 to minimize errors detected and corrected in recoveredrandomized data 382 by decoder 384, for example.

FIG. 6 is a flowchart illustrating an exemplary method 600 of thisinvention for transferring data through signal channel 306. Method 600begins at the step 602 by first selecting a two-dimensional (2-D)constellation of symbols distributed on the phase plane asymmetricallyabout the origin, such as constellation 400 or constellation 500discussed above in connection with FIGS. 5A-B, for example. Next, at thestep 604, the incoming data are encoded as complex symbols (I, Q)selected from the 2-D constellation, and, in the step 606, first andsecond baseband modulating signals I(t) and Q(t) are produced, whoseamplitudes together represent the time series of complex symbols (I, Q)and at least one of the baseband modulating signals has a substantiallynon-zero mean amplitude. Then, in the step 608, the first basebandmodulating signal I(t) is multiplied by an in-phase baseband symbolclock signal to produce a first modulation product signal as, in thestep 610, the second baseband modulating signal Q(t) is multiplied by aquadrature baseband symbol clock signal to produce a second modulationproduct signal. In the step 612, the first and second modulation productsignals are summed to produce a transmitter output signal, which iscoupled through the signal channel to the data receiver in the step 614.Finally, in the step 616, the two modulation product signals aredemodulated at the data receiver to recover the series of complexsymbols (I, Q), thereby facilitating recovery of the incoming data (notshown).

Improving QAM Bit Error Rate (BER) Performance:

The Type III (square) 2-D symbol constellation known in the art andexemplified by constellation 100 (FIG. 1), is disposed so that themodulating signal amplitudes are symmetrical around zero (phase planeorigin 110), as are all other 2-D QAM symbol constellations of any type.This is a well-known QAM system requirement arising from the universaland well-founded belief that QAM communication system BER performance isdiminished when any power is “wasted” in a carrier (baseband symbolclock) signal. As is known in the art, adding sufficiently exotic (andexpensive) timing recovery means to the QAM receiver can overcome muchof the timing recovery problem arising from the complete suppression ofthe carrier (baseband symbol clock) signal and thereby avoid most of theBER performance penalty arising from baseband symbol clock recoveryerror. This situation, and the unexpectedly advantageous observationleading to the method of this invention, may be better appreciated withreference to the following discussion of the effects of various systemabnormalities on theoretical QAM system BER.

FIG. 7 provides a graph 700 illustrating the theoretical BER undervarious operating conditions for several 256-QAM communications systemembodiments from the prior art. The BER curve 702 provides the predictedBER of the ideal theoretical QAM modulator and demodulator embodiment810 shown in FIG. 8A. As shown in FIG. 8A, embodiment 810 includes a QAMmodulator 812 coupled to a QAM demodulator 814 through an ideal signalchannel 816. No actual channel or baseband symbol clock apparatus isshown because theoretically ideal demodulation is assumed for thepurposes of predicting BER curve 702.

In FIG. 7, the BER curve 704 provides the predicted BER of thetheoretical QAM modulator and demodulator embodiment 830 shown in FIG.8B. As shown in FIG. 8B, embodiment 830 includes a QAM modulator 832coupled to a QAM demodulator 834 through an ideal signal channel 836.The original baseband symbol clock signal 838 is assumed to be availableto QAM demodulator 834 with neither distortion nor delay other than theaddition of Additive White Gaussian Noise (AWGN). The 3 dB reduction inperformance in BER curve 704 compared to the ideal baseline BER curve702 is understandable because the four samples per symbol clock cycleassumed for these predictions implies a loss of information otherwiseavailable by integrating out the effects of AWGN.

Returning to FIG. 7, the BER curve 706 provides the predicted BER of theQAM modulator and demodulator embodiment 850 shown in FIG. 8C. As shownin FIG. 8C, embodiment 850 includes a QAM modulator 852 coupled to a QAMdemodulator 854 through a cable and preamplifier signal channel 856. Thebaseband symbol clock timing is recovered at QAM demodulator 854 bymeans of a simple PLL 858. The performance shown by BER curve 706 isdismal because the complete suppression of the baseband symbol clocksignal from QAM modulator 852 makes the reliance on a “simple” PLL 858for clock recovery an unrealistic solution to the clock recoveryproblem.

In FIG. 7, the BER curve 708 provides the predicted BER of the QAMmodulator and demodulator embodiment 870 shown in FIG. 8D. As shown inFIG. 8D, embodiment 870 includes a QAM modulator 872 coupled to a QAMdemodulator 874 through a cable and preamplifier signal channel 876. Thebaseband symbol clock timing is recovered at QAM demodulator 874 bymeans of a complex “exotic” clock recovery means 878. By using anysufficiently sophisticated baseband symbol clock timing recoverymechanism known in the art for the exotic recovery means 878, BER curve708 provides a performance that is no worse than BER curve 704 at higherBER values and no more than 5-6 dB worse at lower BER values. Thisvariation between BER curves 704 and 708 is related to timing andequalization error degradation and is accepted in the art as aperformance sacrifice made to avoid the undesirable performancereduction from “carrier leakage” in QAM systems (FIG. 9).

The effects on BER of an asymmetric QAM constellation may be appreciatedwith reference to FIG. 9. FIG. 9 provides a graph 900 illustrating thetheoretical BER under various operating conditions for two 256-QAMcommunications system embodiments using the exemplary asymmetric symbolconstellations 400 and 500 discussed above (FIGS. 5A-5B). The BER curve902 provides the predicted BER of QAM modulator and demodulatorembodiment 830 discussed above (FIG. 8B) and is identical to BER curve704 in FIG. 7. The BER curves 904 and 906 provide the predicted BER ofthe QAM modulator and demodulator embodiment 1010 shown in FIG. 10Aunder two different conditions. As shown in FIG. 10A, embodiment 1010includes a QAM modulator 1012 coupled to a QAM demodulator 1014 throughan ideal signal channel 1016. The original baseband symbol clock signal1018 is assumed to be provided to QAM demodulator 1014 with neitherdistortion nor delay for the purposes of predicting BER curves 904 and906. A baseband symbol constellation offset 1020 is provided to move the2-D baseband symbol constellation (not shown) with respect to one of thephase plane axes and thereby insert a “power wasting” baseband symbolclock signal in accordance with the method and system of this invention.For BER curve 904, offset 1020 is set to +5.0 units to create symbolconstellation 400 (FIG. 5A) and, for BER curve 906, offset 1020 is setto +8.0 units to create symbol constellation 500 (FIG. 5B).

In FIG. 9, note that offsetting the symbol amplitudes by 5.0 units alongthe I-axis of the phase plane (FIG. 5A) provides the BER curve 904,which shows a BER performance reduction of 2-3 dB with respect to BERcurve 902. Offsetting the symbol amplitudes by another 3.0 units alongthe I-axis of the phase plane (FIG. 5B) provides the BER curve 906,which shows a BER performance reduction of an additional 2-3 dB withrespect to BER curve 904. This “power-wasting” penalty is the well-knownreason why (until now) all 2-D symbol constellations are forced intosymmetry about the phase plane origin. Also, this BER performance lossis consistent with the relative root mean square (RMS) powers containedin the respective time-domain waveforms, as may be appreciated withreference to the following discussion of FIGS. 11-12.

FIG. 11 is a graph illustrating the baseband transmitter output signal1100 in the time domain from 256-QAM system embodiment 200 (FIG. 3)using symmetrical symbol constellation 100 (FIG. 1). FIG. 12 is a graphillustrating the baseband transmitter output signal 1200 in the timedomain from 256-QAM system embodiment 300 (FIG. 4) using asymmetricalsymbol constellation 400 (FIG. 5A). Note that some additional (“wasted”)RMS power is clearly evident in baseband transmitter output signal 1200when compared with baseband transmitter output signal 1100.

But examining these same two baseband transmitter output signals 1100and 1200 in the frequency domain provides additional useful insight intothe baseband symbol clock recovery problem and the method of thisinvention. FIG. 13 provides a baseband transmitter output spectrum 1300illustrating baseband transmitter output signal 1100 (FIG. 11) in thespectral domain and FIG. 14 provides a baseband transmitter outputspectrum 1400 illustrating baseband transmitter output signal 1200 (FIG.12) in the spectral domain. Even though system performance is degradedby 2 dB because of the 2 dB increase in RMS power in basebandtransmitter output signal 1200 over the RMS power in basebandtransmitter output signal 1100, the power at the baseband symbol clockfrequency 1402 in baseband transmitter output spectrum 1400 now risesabove the remainder of the spectrum by about 18 dB compared to the powerat the baseband symbol clock frequency 1302 in baseband transmitteroutput spectrum 1300. This is more than adequate to facilitate a verysimple means for symbol clock timing recovery in the manner nowdiscussed. Note that the two baseband transmitter output spectra 1300and 1400 are substantially identical except for the 18 dB spike at thebaseband symbol clock frequency 1402 (FIG. 14).

FIG. 15 is a graph 1500 illustrating the theoretical BER under variousoperating conditions for 256-QAM communications system embodiments ofthis invention using asymmetric symbol constellation 400 (FIG. 5A). TheBER curve 1502 provides the predicted BER of QAM modulator anddemodulator embodiment 1010 with offset 1020 set to +5.0 units and isidentical to curve 904 from FIG. 9.

In FIG. 15, the BER curve 1504 provides the predicted BER of the QAMmodulator and demodulator embodiment 1030 shown in FIG. 10B. As shown inFIG. 10B, embodiment 1030 includes a QAM modulator 1032 coupled to a QAMdemodulator 1034 through a cable and preamplifier signal channel 1036.The original baseband symbol clock signal 1038 is assumed to be providedto QAM demodulator 1034 with delay only and no distortion. Theperformance of BER curve 1504 is not significantly different from BERcurve 1502 because the delayed but otherwise unaffected baseband symbolclock signal is also available at QAM demodulator 1034.

In FIG. 15, the BER curve 1506 provides the predicted BER of the QAMmodulator and demodulator embodiment 1050 shown in FIG. 10C and isgenerally indistinguishable from BER curve 1502 because of theadvantageous effects of the asymmetric symbol constellation 400 (FIG.5A) used in accordance with the method of this invention. As shown inFIG. 10C, embodiment 1050 includes a QAM modulator 1052 coupled to a QAMdemodulator 1054 through a cable and preamplifier signal channel 1056.The baseband symbol clock timing is recovered at QAM demodulator 1054 bymeans of a simple PLL 1058. Curve 1504 BER performance is notsignificantly different from curve 1502 because the 18 dB spike at thebaseband symbol clock frequency 1402 (FIG. 14) permits the reliance on a“simple” PLL 1058 for effective clock recovery, for the first time.

Note that the advantages of the method of this invention may beappreciated by comparing BER curve 706 (FIG. 7) to BER curve 1506 (FIG.15). Although both examples use simple PLL baseband symbol clockrecovery, the performance of BER curve 1506 demonstrates that there isno additional timing recovery penalty. Timing can be recovered withoutappreciable performance loss using the simple and inexpensive recoverymeans exemplified by PLL 1058 (FIG. 10C).

And there are additional benefits as well, including the availability ofthe large single frequency spike at the baseband symbol clock frequency1402 (FIG. 14) for predicting abnormalities in signal channel 306 (FIG.4). Referring to FIG. 4, this channel prediction capability facilitatesthe simplification of signal conditioner 344, which represents the onlyremaining element of QAM communications system 300 having anysignificant complexity or expense. Recall that the remainder of basebandsymbol clock detector 350 and all other remaining complexity in QAM datareceiver 304 are embodied within the simple and inexpensive DSP 359.

This asymmetric symbol constellation technique differs significantlyfrom and avoids several disadvantages (e.g., increased signal envelopefluctuation and spectral spreading) of a concept for inserting aseparate tone in the transmitted signal to facilitate measurement ofsignal channel characteristics that is sometimes denominatedTransparent-Tone-In-Band (TTIB) modulation. The TTIB concept neitherconsiders nor suggests using a simple offset signal to shift thebaseband symbol clock constellation about the phase plane as describedabove. TTIB requires the creation of a separate tone and insertion intothe channel in the communications band. The separate tone must then beremoved somehow from the received signal before attempting demodulationand decoding. This adds complexity and expense to the communicationssystem rather than reducing complexity. The TTIB modulation may becharacterized as offsetting the baseband symbol clock signal in timeinstead of offsetting the baseband symbol constellation in amplitude onthe phase plane and results in generating overlapping sidebands, therebyaltering the frequency spectrum and bandwidth of the transmitted signal.This introduces additional well-known problems that may be appreciatedwith reference to, for example, McGeehan et al. [“Phase-LockedTransparent Tone In Band (TIIB): A new spectrum configurationparticularly suited to the transmission of data over SSB mobile radionetworks,” IEEE Transactions on Communications, vol COM32, 1984] andHanzo et al. [“Quadrature Amplitude Modulation,” Second Edition, IEEEPress, 2004, John Wiley].

Finally, the utility and advantage of the method of this invention maybe best appreciated with reference to FIG. 16, which is a graph 1600comparing BER curves 704 and 708 from FIG. 15 to BER curves 1502 and1506 from FIG. 7. Recall that BER curve 704 provides the system BERperformance assuming perfect recovery of the original baseband symbolclock signal 838 at QAM demodulator 834 (FIG. 8B). And BER curve 708provides just about the best system BER performance known in the QAM artfor a real signal channel and is obtained only by using anysophisticated baseband symbol clock timing recovery mechanism known inthe art for exotic recovery means 878 (FIG. 8D). Note that, compared toBER curve 708, BER curves 1502 and 1506 both show superior BERperformance below the BER value represented by a crossover point 1602(about 2E-04 to 3E-04 in this example) and falls only 1-2 dB behind BERcurve 708 at the BER values well above crossover point 1602. In otherwords, the method and system of this invention improves BER performanceover the QAM prior art in any application operating beyond crossoverpoint 1602 (SNR=about 26 dB in this example) and does this withsubstantially less complexity and expense.

By offsetting the 2-D baseband symbol constellation with respect to thephase plane origin, symbol clock leakage is inserted into thetransmitted QAM signal. While this slightly degrades static BERperformance alone, this discussion discloses for the first time that theasymmetrical constellation actually improves overall system performancewhen considering baseband symbol clock recovery and received signalcompensation for an imperfect signal channel. This improvement, for thefirst time, allows QAM to be deployed in systems where QAM is otherwiseprohibitively expensive. This improvement, for the first time, alsoallows overall system per-tem performance to be improved for anyexisting QAM system without additional bandwidth, cost or complexity.

A Pipe Inspection System Embodiment

Advantageously, the QAM system and method of this invention may beembodied in a video transmitter to send high definition video signal upa pipe-inspection system cable to a video receiver. This QAM videosignal does not interfere with data link and other cable uses in thepipe-inspection system. For example, the QAM video signal does not usebandwidth near 32 kHz or 512 Hz, so it does not suffer from interferencefrom the system's sonde (512 Hz) or tracer frequency (32,768 Hz). Thisembodiment provides performance superior to a standard NTSC signal,which is degraded by the cable, offers less picture quality, andinterferes with sonde and/or tracer operation.

FIG. 17 is a perspective diagram illustrating an exemplary pipeinspection system embodiment 1701 incorporating the data transfer systemand method of this invention. Referring to FIG. 17, a pipe inspectionsystem 1701 includes a camera head 1713 operatively connected to thedistal end of a push-cable 1709. The proximal end of the push-cable 1709is operatively connected to a cable-counter and user interface panel1705 through a slip-ring assembly 1707. Examples of suitableconstructions for the camera head 1713 are disclosed in U.S. Pat. No.6,831,679 entitled “Video Camera Head with Thermal Feedback Control,”granted to Mark S. Olsson et al. on Dec. 14, 2004, and in U.S. patentapplication Ser. No. 10/858,628 entitled “Self-Leveling Camera Head,” ofMark S. Olsson filed Jun. 1, 2004, the entire disclosures of which arehereby incorporated by reference. Push-cable constructions andtermination assemblies suitable for use in connecting the proximal anddistal ends of a push-cable are disclosed in U.S. Pat. No. 5,939,679entitled “Video Push Cable” granted Aug. 17, 1999 to Mark S. Olsson,U.S. Pat. No. 6,958,767 entitled “Video Pipe Inspection System EmployingNon-Rotating Cable” granted Oct. 25, 2005, to Mark S. Olsson et al.,U.S. patent application Ser. No. 12/371,540 filed Feb. 13, 2009 entitled“Push-Cable for Pipe Inspection System,” and U.S. Patent ApplicationSer. No. 61/152,947 filed Feb. 16, 2009 by Mark S, Olsson et al.entitled “Pipe Inspection System with Replaceable Cable Storage Drum,”the entire disclosures of which are hereby incorporated by reference. InFIG. 17, a reel 1703 holds coils of the push-cable 1709. The push-cable1709 is paid out from reel 1703 to force camera head 1713 down pipe1711. Examples of a suitable reel 1703 and push-cable 1709 are disclosedin the aforementioned U.S. Pat. No. 6,958,767. Within the reel 1703, aslip-ring assembly 1707 provides rotary signals to an associated circuitboard (not shown) which enables them to be translated into digitalmeasurements of distance traversed by the push-cable 1709 based on therotation of the drum. One example of a suitable slip ring assembly isdisclosed in U.S. Pat. No. 6,908,310 entitled “Slip Ring Assembly withIntegral Position Encoder,” granted Jun. 21, 2005, to Mark S. Olsson etal., the entire disclosure of which is hereby incorporated by reference.The camera head 1713 with its on-board circuitry transmits imageinformation through embedded conductors such as wires in the push-cable1709. A display unit 1715 shows the updated field of view (FOV) imagefrom the camera head 1713 with an overlay indicating the distancedown-pipe and the direction of travel based on the values transmittedfrom the slip ring assembly 1707. Circuit boards within theuser-interface assembly 1705 provide memory and processing, userinformation display and input controls.

Turning now to FIG. 18, the electronic portion 1800 of pipe inspectionsystem 1701 includes a central processor 1802 associated with a volatilememory 1818, which receives input data 1819 from a user interface 1806,a slip-ring counter 1808, a remote video camera 1804 including a videotransmitter 1825, which incorporates the elements of QAM datatransmitter 302 substantially as shown in FIG. 4 and operatingsubstantially as discussed above. Video transmitter 1825 providing avideo signal 1823 representing image data passing through a signalchannel 1827 to a video receiver 1824, which incorporates the elementsof QAM data receiver 304 substantially as shown in FIG. 4 and operatingsubstantially as discussed above. Signal channel 1827 is embodied as oneor more electrical conductors disposed within push-cable 1709 (FIG. 17).Central processor 1802 is also associated with camera control circuitry1814, a system graphical user interface (GUI) 1826, and a keyboard 1820.The central processor 1802 sends output signals to the camera control1814, volatile memory 1818, SD card storage 1810, USB portable (thumbdrive) storage 1812, and the user GUI 1826 with its associated display1828, which also displays images 1829 responsive to video signal 1823arriving at video receiver 1824 upon proper user or software command.The transfer of image and other data may be automated through firmwareprogramming or initiated from the GUI 1826 using on-board key presses,or by means of the keyboard 1820. Algorithmic options in the firmwaremay permit parameters such as distance interval between image captures,for example, to be set to default values in automatic operation or to beset to user selected values using menu options exercised through UI 1826or keyboard 1820.

FIG. 19 is a flowchart illustrating an exemplary method 1900 of thisinvention for transferring video signal 1823 through signal channel 1827in electronic portion 1800 of pipe inspection system 1701. Method 1900begins at the step 1902 by first selecting a two-dimensional (2-D)constellation of symbols distributed on the phase plane asymmetricallyabout the origin, such as constellation 400 or constellation 500discussed above in connection with FIGS. 5A-B, for example. Next, at thestep 1904, the video signal data are encoded as complex symbols (I, Q)selected from the 2-D constellation, and, in the step 1906, first andsecond baseband modulating signals I(t) and Q(t) are produced, whoseamplitudes together represent the time series of complex symbols (I, Q)and at least one of the baseband modulating signals has a substantiallynon-zero mean amplitude. Then, in the step 1908, the first basebandmodulating signal I(t) is multiplied by an in-phase baseband symbolclock signal to produce a first modulation product signal as, in thestep 1910, the second baseband modulating signal Q(t) is multiplied by aquadrature baseband symbol clock signal to produce a second modulationproduct signal. In the step 1912, the first and second modulationproduct signals are summed to produce a transmitter output signal, whichis coupled through the signal channel to the data receiver in the step1914. Finally, in the step 1916, the two modulation product signals aredemodulated at the data receiver to recover the series of complexsymbols (I, Q), thereby facilitating recovery of the video signal data(not shown).

Clearly, other embodiments and modifications of this invention may occurreadily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawing.

We claim:
 1. In a communication system including a data transmittercoupled through a signal channel to a data receiver, a method fortransferring data through the signal channel comprising the steps of:encoding the data to produce a first baseband modulating signal I(t) anda second baseband modulating signal Q(t) whose amplitudes togetherrepresent a time series of complex symbols (I, Q) each selected from atwo-dimensional (2-D) constellation of symbols distributed on the phaseplane about the origin such that at least one of the baseband modulatingsignals has a substantially non-zero mean amplitude; multiplying thefirst baseband modulating signal I(t) by a first baseband symbol clocksignal to produce a first modulation product signal and multiplying thesecond baseband modulating signal Q(t) by a second baseband symbol clocksignal to produce a second modulation product signal, where the phasesof the first and second baseband symbol clock signals are generallyfixed in quadrature; summing the first and second modulation productsignals to produce a transmitter output signal; coupling the transmitteroutput signal through the signal channel to the data receiver; anddemodulating the first and second modulation product signals at the datareceiver to recover the series of complex symbols (I, Q).
 2. The methodof claim 1 wherein the 2-D symbol constellation is square and disposedasymmetrically about one of the two phase plane axes.
 3. The method ofclaim 1 wherein the signal channel is selected from a group consistingessentially of an electrical conductor, an optical fiber and afree-space electromagnetic wave propagation path.
 4. The method of claim1 further comprising the steps of: detecting the first baseband symbolclock signal from the transmitter output signal at the data receiver;and producing a recovered baseband symbol clock signal that issynchronized with the first baseband symbol clock signal at the datareceiver.
 5. A communication system comprising: a data transmitter,including an input for accepting data, a Quadrature Amplitude Modulation(QAM) encoder coupled to the data input for producing, responsive to thedata, a first baseband modulating signal I(t) and a second basebandmodulating signal Q(t) whose amplitudes together represent a time seriesof complex symbols (I, Q) each selected from a two-dimensional (2-D)constellation of symbols distributed on the phase plane about the originsuch that at least one of the baseband modulating signals has asubstantially non-zero mean amplitude, a QAM modulator coupled to theQAM encoder for multiplying the first baseband modulating signal I(t) bya first baseband symbol clock signal to produce a first modulationproduct signal and multiplying the second baseband modulating signalQ(t) by a second baseband symbol clock signal to produce a secondmodulation product signal, where the phases of the first and secondbaseband symbol clock signals are generally fixed in quadrature, and forsumming the first and second modulation product signals to produce atransmitter output signal, and an output for coupling the transmitteroutput signal to a signal channel; and a data receiver, including asignal input coupled to the signal channel for accepting the transmitteroutput signal, and a QAM demodulator coupled to the signal input forrecovering the series of complex symbols (I, Q) from the first andsecond modulation product signals.
 6. The system of claim 5 wherein the2-D symbol constellation is square and disposed asymmetrically about oneof the two phase plane axes.
 7. The system of claim 5 wherein the signalchannel is selected from a group consisting essentially of an electricalconductor, an optical fiber and a free-space electromagnetic wavepropagation path.
 8. The system of claim 5 further comprising: in thedata receiver, a baseband symbol clock detector coupled to the signalinput for detecting the first baseband symbol clock signal from thetransmitter output signal; and a baseband symbol clock recoveryoscillator coupled to the baseband symbol clock detector for producing arecovered baseband symbol clock signal synchronized with the firstbaseband symbol clock signal.
 9. In a communication system fortransferring data through a signal channel to a data receiver, a datatransmitter comprising: an input for accepting data; a QuadratureAmplitude Modulation (QAM) encoder coupled to the data input forproducing, responsive to the data, a first baseband modulating signalI(t) and a second baseband modulating signal Q(t) whose amplitudestogether represent a time series of complex symbols (I, Q) each selectedfrom a two-dimensional (2-D) constellation of symbols distributed on thephase plane about the origin such that at least one of the basebandmodulating signals has a substantially non-zero mean amplitude; a QAMmodulator coupled to the QAM encoder for multiplying the first basebandmodulating signal I(t) by a first baseband symbol clock signal toproduce a first modulation product signal and multiplying the secondbaseband modulating signal Q(t) by a second baseband symbol clock signalto produce a second modulation product signal, where the phases of thefirst and second baseband symbol clock signals are generally fixed inquadrature, and for summing the first and second modulation productsignals to produce a transmitter output signal; and an output forcoupling the transmitter output signal through the signal channel to thedata receiver.
 10. The data transmitter of claim 9 wherein the 2-Dsymbol constellation is square and disposed asymmetrically about one ofthe two phase plane axes.
 11. The data transmitter of claim 9 whereinthe signal channel is selected from a group consisting essentially of anelectrical conductor, an optical fiber and a free-space electromagneticwave propagation path.
 12. In a remote inspection system including avideo transmitter coupled through a signal channel to a video receiver,a method for transferring a video signal through the signal channelcomprising the steps of: encoding the video data to produce a firstbaseband modulating signal I(t) and a second baseband modulating signalQ(t) whose amplitudes together represent a time series of complexsymbols (I, Q) each selected from a two-dimensional (2-D) constellationof symbols distributed on the phase plane about the origin such that atleast one of the baseband modulating signals has a substantiallynon-zero mean amplitude; multiplying the first baseband modulatingsignal I(t) by a first baseband symbol clock signal to produce a firstmodulation product signal and multiplying the second baseband modulatingsignal Q(t) by a second baseband symbol clock signal to produce a secondmodulation product signal, where the phases of the first and secondbaseband symbol clock signals are generally fixed in quadrature; summingthe first and second modulation product signals to produce a transmitteroutput signal; coupling the transmitter output signal through the signalchannel to the data receiver; and demodulating the first and secondmodulation product signals at the data receiver to recover the series ofcomplex symbols (I, Q).
 13. The method of claim 12 wherein the 2-Dsymbol constellation is square and disposed asymmetrically about one ofthe two phase plane axes.
 14. The method of claim 12 wherein the signalchannel is selected from a group consisting essentially of an electricalconductor, an optical fiber and a free-space electromagnetic wavepropagation path.
 15. The method of claim 12 further comprising thesteps of: detecting the first baseband symbol clock signal from thetransmitter output signal at the video receiver; and producing arecovered baseband symbol clock signal that is synchronized with thefirst baseband symbol clock signal at the video receiver.
 16. In aremote inspection system including a video transmitter coupled through asignal channel to a video receiver, a data modulator in the videotransmitter for transferring a video signal through the signal channel,the data modulator comprising: an input for accepting data; a QuadratureAmplitude Modulation (QAM) encoder coupled to the data input forproducing, responsive to the data, a first baseband modulating signalI(t) and a second baseband modulating signal Q(t) whose amplitudestogether represent a time series of complex symbols (I, Q) each selectedfrom a two-dimensional (2-D) constellation of symbols distributed on thephase plane about the origin such that at least one of the basebandmodulating signals has a substantially non-zero mean amplitude; and aQAM modulator coupled to the QAM encoder for multiplying the firstbaseband modulating signal I(t) by a first baseband symbol clock signalto produce a first modulation product signal and multiplying the secondbaseband modulating signal Q(t) by a second baseband symbol clock signalto produce a second modulation product signal, where the phases of thefirst and second baseband symbol clock signals are generally fixed inquadrature, and for summing the first and second modulation productsignals to produce a transmitter output signal.
 17. The data modulatorof claim 16 wherein the 2-D symbol constellation is square and disposedasymmetrically about one of the two phase plane axes.
 18. A remoteinspection system comprising: a video transmitter, including, a videocamera for producing video data, and a Quadrature Amplitude Modulation(QAM) coupled to the video camera, including a symbol encoder coupled tothe video camera for producing, responsive to the video data, a firstbaseband modulating signal I(t) and a second baseband modulating signalQ(t) whose amplitudes together represent a time series of complexsymbols (I, Q) each selected from a two-dimensional (2-D) constellationof symbols distributed on the phase plane about the origin such that atleast one of the baseband modulating signals has a substantiallynon-zero mean amplitude, a QAM modulator coupled to the QAM encoder formultiplying the first baseband modulating signal I(t) by a firstbaseband symbol clock signal to produce a first modulation productsignal and multiplying the second baseband modulating signal Q(t) by asecond baseband symbol clock signal to produce a second modulationproduct signal, where the first and second baseband symbol clock signalsare generally fixed in quadrature, a summer coupled to the QAM modulatorfor summing the first and second modulation product signals to produce atransmitter output signal, and a signal output for coupling thetransmitter output signal to a signal channel; and a video receiver,including a video signal input coupled to the signal channel forproducing a receiver input signal responsive to the transmitter outputsignal, a QAM demodulator coupled to the video signal input forrecovering the video data from the receiver input signal, and a videodisplay coupled to the QAM demodulator for producing images responsiveto the video data.
 19. The system of claim 18 wherein the 2-D symbolconstellation is square and disposed asymmetrically about one of the twophase plane axes.
 20. The system of claim 18 wherein the signal channelis selected from a group consisting essentially of an electricalconductor, an optical fiber and a free-space electromagnetic wavepropagation path.
 21. The system of claim 18 further comprising: in thevideo receiver, a baseband symbol clock detector coupled to the videosignal input for detecting the first baseband symbol clock signal fromthe receiver input signal; and a baseband symbol clock recoveryoscillator coupled to the baseband symbol clock detector for producing arecovered baseband symbol clock signal synchronized with the firstbaseband symbol clock signal.
 22. A pipe inspection system comprising: avideo transmitter, including a video camera adapted to produce videodata, and a Quadrature Amplitude Modulation (QAM) modulator coupled tothe video camera, including a symbol encoder for producing, responsiveto the video data, a first baseband modulating signal I_(T(t)) and asecond baseband modulating signal Q_(T(t)) whose amplitudes togetherrepresent a time series of complex transmitter symbols (I_(T), Q_(T))each selected from a two-dimensional (2-D) constellation of symbolsdistributed on the phase plane about the origin such that at least oneof the baseband modulating signals has a substantially non-zero meanamplitude, a baseband symbol clock oscillator for producing first andsecond baseband symbol clock signals generally fixed in quadrature, adual multiplier coupled to the symbol encoder and baseband symbol clockoscillator for multiplying the first baseband modulating signal I_(T(t))by the first baseband symbol clock signal to produce a first modulationproduct signal and for multiplying the second baseband modulating signalQ_(T(t)) by the second baseband symbol clock signal to produce a secondmodulation product signal, a summer coupled to the dual multiplier forsumming the first and second modulation product signals to produce atransmitter output signal, and a filter coupled to the summer forproducing a filtered transmitter output signal; a mechanical cableassembly coupled to the video transmitter for urging the videotransmitter through a pipe under inspection and including an electricalconductor coupled to the QAM modulator for accepting the filteredtransmitter output signal; and a video receiver, including a signalconditioner coupled to the electrical conductor for producing a basebandreceiver input signal representing the filtered transmitter outputsignal, a QAM demodulator coupled to the signal conditioner, including abaseband symbol clock detector for detecting the first baseband symbolclock signal from the baseband receiver input signal, a baseband symbolclock recovery oscillator coupled to the baseband symbol clock detectorfor producing a first recovered baseband symbol clock signal generallysynchronized with the first baseband symbol clock signal and forproducing a second recovered baseband symbol clock signal generallyfixed in quadrature with the first recovered baseband symbol clocksignal, a dual multiplier coupled to the baseband symbol clock recoveryoscillator for multiplying the baseband receiver input signal by thefirst and second recovered baseband symbol clock signals to producefirst and second demodulation product signals, respectively, a dualfilter coupled to the dual multiplier for producing, responsive to thefirst and second demodulation product signals respectively, first andsecond baseband demodulated signals, I_(R(t)) and Q_(R(t)), whoseamplitudes together represent a time series of complex receiver symbols(I_(R), Q_(R)), and a decoder coupled to the QAM demodulator forrecovering the video data from the first and second demodulated signals,I_(R(t)) and Q_(R(t)), and a video display coupled to the QAMdemodulator for producing images responsive to the video data.
 23. Thesystem of claim 22 wherein the 2-D symbol constellation is square anddisposed asymmetrically about one of the two phase plane axes.